This grant application addresses the mucosal innate immune recognition and response to the flagellated mucosal pathogen, Salmonella typhimurium. The innate immune response is the product of both multiple cell types and multiple receptors;this combination of events results in a tailored" inflammatory response to the pathogen. We hypothesize that innate immune receptors have specific and redundant functions that regulate a plethora of outputs, which are responsible for controlling bacterial infection. Toll-like receptors (TLR) constitute a prototypic family of innate immune receptorsthat recognize specific microbial components. We identified bacterial flagellin as the ligand for TLR5, defined the TLR5 recognition site on flagellin as a highly conserved region that is required for bacterial motility, demonstrated physical association of TLR5 and flagellin, and recently characterizedtwo major subclasses of bacteria that have evaded TLR5 recognition. Our studies and those of others suggest that TLR5 is a critical receptor for innate immune defense against flagellated pathogens at mucosal surfaces. The NOD family of immune receptors recognizesbacterial peptidoglycans, and we present data demonstrating that NOD1 recognizes S. typhimurium and synergistically interacts with TLRs. Although a detailed model of ligand recognition and signaling is emerging for TLRS and NOD1, little is know about the significance of these innate immune receptorsin vivo. In this proposal we aim to extend our studies into in vivo models of bacterial infection, using Salmonella as the model pathogen. Our efforts are focused on the roles of TLRS and NOD1 in regulating inflammatory responses and bacterial colonization during the early phases of oral infection. We will use in vivo and directed in vitro models of Salmonella infection to determine specific contributions of TLRS recognition of bacterial flagellin (Aim 1), NOD1 recognition of diaminopimelic acid containing peptidoglycans (Aim 2), and specific host cell types (Aim3) to mucosal innate defense against Salmonella.